HDTDX - DTX CableAnalyzer

• DTX-CU-201 Diagnosing a Category 6 NEXT failure #1 (Badly terminated permanent link)
• DTX-CU-202 Diagnosing a Category 6 NEXT failure #2 (Cable Issue

​​The HDTDX (High Definition Time Domain Xtalk) features of the DTX and DSX CableAnalyzers show test results in the Time Domain rather than the Frequency Domain and can be used to locate the distance to NEXT events.  The following links give guidance on how to perform this troubleshooting using the DTX and DSX CableAnalyzers.

• NEXT Fails on DTX Cable Analyzer due to a termination issue
• NEXT Fails on DSX Cable Analyzer due to a termination issue

The NEXT measurements provided by the DTX CableAnalzyer are requirements of the standard and are reproducible against laboratory equipment; however the HDTDX is a tool to assist the user with fault location and opportunity to improve link performance.

Use of these HDTDX fault finding capabilities to determine exact performance of an individual connector within a link can be difficult, and is variable due to the length, topology of a link and performance of the other components within that link. However, did you know that the HDTDX trace can be used to estimate the performance of an individual connector in a link? 
 

Here we see an HDTDX trace taken from a Category 5e link which had a NEXT margin of 2.4 dB. It is a pass, but for this particular connector and cable vendor we would expect to see larger margins for this particular configuration. The HDTDX trace suggests that the connectors are the issue. The DTX reports the peak HDTDX event to be '25.1'. What does 25.1 represent? Firstly, that '25.1' is an HDTDX value of 25.1%. 

Assuming the manufacturers claim of component compliance to the standard, FNET have established that any DTX HDTDX event at the connector locations greater than approximately 17.5% is a potential cause of concern for links longer than 15m, and may be improved by re-termination.

Since the test plug in the permanent link adapter is guaranteed to be TIA, ISO and EN compliant for performance, we can estimate component NEXT performance in an installed link by equating an DTX HDTDX result of approximately 17.5 % with the connecting hardware limit at 100 MHz. Something you cannot do using adapterless testing, where ordinary patch cords are used.

The cabling standards define the performance of a mated connector as:

54 dB @ 100 MHz for Category 6

43 dB @ 100 MHz for Category 5e

The TIA and ISO standards have a specific process defined to do component testing for jacks and cable. These component tests are designed to be performed in a laboratory environment and will tell you what the specific performance of the components are.

[DJ1] How can we estimate a connector's performance by looking at just one frequency point? A point source of NEXT, at the location where it is, typically exhibits in the frequency domain a response which has a 20 dB/decade slope. Its intensity can therefore be characterized at any frequency; we chose 100 MHz for ease of use and understanding. It should be noted that the worst case performance of connector may not be at 100MHz but at a different frequency and hence the values of the expected HDTDX result may need to be adjusted.

Doesn’t the effect of crosstalk reduce as you go down the link caused by the signal attenuating (getting smaller)? When the point source of NEXT is some distance away from the near end of the link under test, the signal amplitude is reduced by round-trip insertion loss as well as bandwidth limitations. (Fluke has a patent on this) A correction algorithm in the DTX applies a gain, so the point source of NEXT in the link appears with a similar HDTDX as a function of distance

To reduce the impact of very near end connector (the DTX instrument connector itself and the channel adapter connection),
the HDTDX sensitivity is reduced by a factor of 10.

The following examples of look up tables provide values for the HDTDX. For example, if we equate a HDTDX threshold of 17.5% to the connecting hardware limit at 100MHz , and the HDTDX value is 25% (as in the trace above), first find the row where HDTX % is 20 (Highlighted red). Then move across the line to where the HDTDX % is 5 (Highlighted green). The point at which they intersect is the value of link NEXT @ 100 MHz (Highlighted Blue). 

The formulae for this table is:

dB@100MHz = -20*log10(HDTDX/Threshold) + ConnLimit

where ConnLimit is in dB, and HDTDX and Threshold are in percent